Showing papers in "Physica E-low-dimensional Systems & Nanostructures in 2008"

On the performance of supercapacitors with electrodes based on carbon nanotubes and carbon activated material—A review

Abstract: Supercapacitors or electrochemical double-layer capacitors (EDLCs) have capacitance value up to thousands of Farads at the same size as for conventional capacitors. At such capacitance value EDLCs are of interest for electrical energy storage. The specific energy of commercial supercapacitors is limited to 5–6 Wh/kg, whereas for batteries the lower limit is 35–40 Wh/kg. Nonetheless other advantages of supercapacitors make them already useful in conjunction with batteries in power applications. Main results related to supercapacitor performance improvement available in literature are presented. Research efforts have been done to increase the specific capacitance of supercapacitor electrodes based on activated or porous carbon material, already used in commercial products. By using available activated carbon with a specific surface area reaching 3000 m 2 /g, specific capacitance values up to 300 F/g have been reported for the investigated experimental supercapacitors. Nonetheless, further optimization of activated carbon properties and its use in supercapacitor electrodes is required for 300 F/g and higher value. By addition of metallic oxides or conductive polymers in the activated carbon used for EDLC electrodes, specific capacitance enhancement takes place. Carbon nanotubes used in experimental supercapacitor electrodes resulted in specific capacitance as high as 180 F/g but higher electrical conductivity and consequently, specific power than in the case of activated carbon was observed. Addition of a small percent of carbon nanotubes in the activated carbon for electrodes results in performance improvement (higher capacitance and conductivity). Nevertheless, high cost of carbon nanotubes prevents their use in commercial products.

Sound wave propagation in single-walled carbon nanotubes using nonlocal elasticity

Abstract: Based on the Bernoulli–Euler and Timoshenko beam theories, a single-elastic beam model using nonlocal elasticity is developed for the wave propagation in carbon nanotubes (CNTs). The small-scale effect is taken into consideration in the present theory. Frequency equations and modal shape functions of Timoshenko beams structures with some typical boundary conditions are also derived from nonlocal elasticity. In addition, the applicability of the two beam models is explored by numerical simulations. The research work reveals the significance of the small-scale effect on wave propagation in single-walled CNTs.

The thermal effect on vibration and instability of carbon nanotubes conveying fluid

Abstract: Based on the theory of thermal elasticity mechanics, an elastic Bernoulli–Euler beam model is developed for vibration and instability analysis of fluid-conveying single-walled carbon nanotubes (SWNTs) considering the thermal effect. Results are demonstrated for the dependence of natural frequencies on the flow velocity as well as temperature change. The influence of temperature change on the critical flow velocity at which buckling instability occurs is investigated. It is concluded that the effect of temperature change on the instability of SWNTs conveying fluid is significant.

Influence of the polymer structure and nanotube concentration on the conductivity and rheological properties of polyethylene/CNT composites

Abstract: A series of multi-walled carbon nanotube /polyethylene (MWNT/PE) composites with several concentrations (0.5, 1, 2.5, 5, 7 wt%) of chemical vapour deposition (CVD)-grown carbon nanotubes (CNTs) have been investigated. High-density polyethylene (HDPE) and low-density polyethylene (LDPE) have been chosen as matrix. The nanocomposites were prepared by melt mixing; a good dispersion in the matrix and a good CNT–polymer interfacial adhesion have been verified by scanning electron microscopy (SEM). In Raman measurements the characteristic bands of the MWNTs are shifted to slightly higher wavenumbers when increasing the CNT content in the nanocomposite, indicating an effective interaction between MWNTs and polymer matrix. Melt rheological investigations in dynamic mode indicate the dispersion degree and the percolation state of the CNTs within the polymer matrix. The rheological percolation threshold of the nanocomposites is between 1 and 2.5 wt%. For HDPE/CNT as well as for LDPE/CNT composites, we found a six orders of magnitude increase in electrical conductivity from 1 to 2.5 wt%, that is the same percolation threshold as determined by rheology. Below percolation threshold we found reproducible diode-like behaviour with different conductivity in forward and reverse bias direction for HDPE sample.

The group III–V's semiconductor energy gaps predicted using the B3LYP hybrid functional

Abstract: Details of the band gaps within semiconductor materials are of paramount importance to a wide range of technological applications. We present the results of an hybrid exchange, B3LYP, approximation to density functional theory for the band gaps of zinc-blend and wurtzite structured III–V materials. Agreement with experimentally derived band gaps at characteristic points in the first Brillouin zone is at least as good as that obtained with correlated calculations, perturbation theories and screened exchange functionals.

Yellow-orange light emission from Mn2+-doped ZnS nanoparticles

Abstract: ZnS nanoparticles with Mn 2 + doping (1–2.5%) have been prepared through a simple soft chemical route, namely the chemical precipitation method. The nanostructures of the prepared undoped ZnS and Mn 2+ - doped ZnS:Mn nanoparticles have been analyzed using X-ray diffraction (XRD), Scanning electron microscope (SEM), transmission electron microscope (TEM) and UV–vis spectrophotometer. The size of the particles is found to be in 2–3 nm range. Room-temperature photoluminescence (PL) spectrum of the undoped sample only exhibits a blue-light emission peaked at ∼365 nm under UV excitation. However, from the Mn 2 + -doped samples, a yellow-orange emission from the Mn 2+ 4 T 1 – 6 A 1 transition is observed along with the blue emission. The prepared 2.5% Mn 2 + -doped sample shows efficient emission of yellow-orange light with the peak emission at ∼580 nm with the blue emission suppressed.

T-shaped channel-drop filters using photonic crystal ring resonators

Abstract: In this paper, we demonstrate a new type of 2D photonic crystal T-shaped channel-drop filters based on ring resonators with high normalized transmission; we investigate parameters which affects resonant frequency in these channel-drop filters. These parameters include dielectric constant of inner rods, coupling rods and whole rods of the structure as well as radius of the coupling rods and scatterer rods. At dropping case, the power transferred to the drop waveguide and the power remaining in the bus waveguide are found by finite difference time domain (FDTD) method to be over 95% and less than 5%, respectively.

Effects of position-dependent effective mass and dielectric function of a hydrogenic donor in a quantum dot

Abstract: Ionization energies of a shallow donor in a quantum dot of GaAs, using a variational procedure within the effective mass approximation, are obtained. Calculations are presented with a constant effective mass and position-dependent effective masses along with the spatially varying dielectric function. Donor binding energies are calculated using both the approximate method ( m 0 * ) and the spatially varying electron effective mass, m * ( r ) . Dielectric quantum dots are discussed and calculations are performed with the dielectric constants of the dot e1 and that of the barrier material, e2. It is found that (i) the use of a constant effective mass (0.067 m0) is justified for dot size ⩾a*, where a* is the effective Bohr radius, which is about 100 A for GaAs, in the estimation of ionization energy, (ii) the ionization energy decreases as the dot increases in both the cases of constant and variable masses, (iii) an increase of binding energy is observed when the spatially varying mass and dielectric function are included, (iv) lower binding energies are observed when the average dielectric constant is included and (v) the binding energy shows complicated behaviour when the position-dependent mass is included for the dot size ⩽a*.

Optical and dielectric properties of PVA capped nanocrystalline PbS thin films synthesized by chemical bath deposition

Abstract: Nanoparticles of lead sulfide (PbS) have been grown within the pores of polyvinyl alcohol (PVA) matrix on glass substrates by chemical bath deposition at and below room temperature (30 °C). Lead acetate and thiourea, dissolved in an alkaline medium, were taken as the sources of lead and sulfur. X-ray diffraction and selected area electron diffraction studies confirmed the cubic nanocrystalline PbS phase formation. Transmission electron micrograph of the films revealed the particle size lying in the range 10–20 nm. X-ray photoelectron spectroscopic studies confirmed the presence of lead and sulfur in the films, and their atomic ratios were found to be dependent on the deposition temperature. UV–vis spectrophotometric measurement showed a direct allowed band gap lying in the range 2.40–2.81 eV, which is much higher than the bulk value (0.41 eV). The band gap decreases with the increase of deposition temperature. The dielectric constant of the PVA-capped nanocrystalline PbS was in the range 155–265 at higher frequencies, which is much higher compared to only PVA and bulk PbS.

Novel optical directional coupler based on surface plasmon polaritons

Abstract: In this paper, finite difference time domain (FDTD) method and perfect matching layer (PML) absorbing boundary condition are adopted to simulate and analyze a novel optical directional coupler (ODC) based on surface plasmon polaritons (SPPs). Transmittance at each output port of the novel ODC with different coupling region lengths shows it follows the general regulations of a conventional ODC. Especially, its transverse size is of nanoscale. The extreme power position offset between the two output ports is proved to be connected with the real part of Ag's complex refractive index. The excess loss and isolation of the ODC are, respectively, 0.57 and 25.9 dB for 1550 nm telecommunication wavelength, when the length of the coupling region equals half of its coupling length.

Scattering mechanisms and Boltzmann transport in graphene

Abstract: Different scattering mechanisms in graphene are explored and conductivity is calculated within the Boltzmann transport theory. We provide results for short-range scattering using the random phase approximation for electron screening, as well as analytical expressions for the dependence of conductivity on the dielectric constant of the substrate. We further examine the effect of ripples on the transport using a surface roughness model developed for semiconductor heterostructures. We find that close to the Dirac point, σ ∼ n β , where β = 1 , 0 ,- 2 for Coulomb, short-range and surface roughness, respectively; implying that Coulomb scattering dominates over both short-range and surface roughness scattering at low density.

Development of carbon nanotube-based gas sensors for NOx gas detection working at low temperature

Abstract: Carbon nanotube (CNT)-based NOx gas sensors which can operate at room temperature were prepared on Al2O3 substrates with interdigitated Pt-electrodes using both dc sputtering method and chemical vapor deposition (CVD) method. In this method, Al buffer layer and Fe catalytic thin film were prepared on the substrate by dc sputtering method and then CNTs were grown by thermal CVD method using ethylene gas. The scanning electron microscope (SEM) images of the CNTs on the substrates indicated that the vertically aligned multi-walled CNT (MWCNT) and the randomly oriented MWCNT were grown selectively by insertion of Al buffer layer. Gas sensing property to NO and NO2 gases were measured. Resistance of the prepared CNT-based gas sensor decreased with increase of NO and NO2 gas concentration. UV light irradiation was examined to detach the adsorbed gas molecule at room temperature. In this paper, it is suggested that CNT-based gas sensors have a great possibility to apply innovative NOx gas sensor from the experimental result.

Graphene Nanoribbon and Graphene Nanodisk

Abstract: We study electronic properties of graphene derivatives which have closed edges. They are finite-length graphene nanoribbons and graphene nanodisks. No metallic states are found in finite-length zigzag nanoribbons though all infinite-length zigzag nanoribbons are metallic. We also study hexagonal, parallelogrammic and trigonal nanodisks with zigzag or armchair edges. No metallic states are found in these nanodisks either except trigonal zigzag nanodisks. It is interesting that we can design the degeneracy of the metallic states arbitrarily in trigonal zigzag nanodisks by changing the size.

Influence of oxygen argon ratio on the structural, electrical, optical and thermoelectrical properties of Al-doped ZnO thin films

Abstract: In this study, high quality of Al-doped ZnO (AZO) thin films were deposited at different oxygen argon ratio by direct current (DC) reactive magnetron sputtering using a Zn target (99.99%) containing Al of 1.5%. The obtained films were characterized and analyzed by X-ray diffraction (XRD) and ultraviolet–visible and infrared light spectrophotometer. The electrical properties had been investigated by van der Pauw method. It was also found that oxygen argon (O 2 /Ar) ratio had great influence on the properties. The results show that AZO thin films are polycrystalline with a preferred (0 0 2) orientation. The film stress increases with increasing O 2 /Ar ratio. The lowest resistivity of 1.306×10 −3 Ω cm was obtained for the AZO thin films prepared at O 2 /Ar ratio of 0.3/27. Hall mobility decreases with increasing the O 2 /Ar ratio. With increasing O 2 /Ar ratio, the transmittance of the AZO thin films has no evident changes. There is a strong absorption in the ultraviolet region. The optical absorption edge is found to shift to the longer wavelength with the increase of O 2 /Ar ratio. The measurements show that there is a striking Seebeck effect in the AZO thin films, and their thermoelectromotive force is linearly increased with increasing temperature difference (Δ T ). With increasing O 2 /Ar ratio and resistance of samples, thermoelectric power (TEP) decreases.

Photoluminescence and Raman analysis of ZnO nanowires deposited on Si(100) via vapor-liquid-solid process

Abstract: ZnO nanowires were deposited on the Si(1 0 0) substrate via vapor–liquid–solid process with flowing Ar gas current for 90 s. The morphology, structure, and optical properties were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL), and Raman spectrum, respectively. The results showed that the as-deposited ZnO nanowires had hexagonal wurzite structure. The Raman spectrum showed oxygen defects in ZnO nanowires due to the existence of the Ar gas during the growth process, leading to the dominant green band peak and the weak UV peak in the PL spectrum. And blue shift of the Raman peaks was attributed to the lattice distortion and piezoelectric effect of the nanostructures. Finally, the biaxial compressive stress within the c -axis oriented ZnO nanowires was estimated to 0.365 GPa, which was also responsible for the frequency shift of the E 2 (high) mode of the Raman spectra.

Relevance of Timoshenko-beam model to microtubules of low shear modulus

Abstract: Microtubules are characterized by extremely low shear modulus that is a few orders of magnitude lower than longitudinal modulus. In this paper, the effects of transverse shearing due to low shear modulus of microtubules are investigated using a Timoshenko-beam model, with detailed comparison between the Timoshenko-beam model, classical isotropic Euler–Bernoulli beam model and a more accurate 2D orthotropic elastic shell model. It is confirmed that transverse shearing is mainly responsible for the length-dependent flexural rigidity of an isolated microtubule reported in the literature, which cannot be explained by the widely used Euler–Bernoulli beam model. Indeed, the length-dependent flexural rigidity predicted by the Timoshenko-beam model is found to be in good quantitative agreement with known experimental data. In particular, the present Timoshenko-beam model predicts that, because of the length dependence of flexural rigidity, microtubules of different lengths could sustain almost equal maximum axial compressive force against column buckling, a conclusion that could have some interesting consequences to the mechanical behavior of cells. These results recommend that the Timoshenko-beam model offers a unified simple 1D model, which can capture the length dependence of flexural rigidity and be applied to various static and dynamic problems of microtubule mechanics.

Humidity-dependent characteristics of methyl-red thin film-based Ag/methyl-red/Ag surface-type cell

Abstract: This paper describes the experimental results for humidity-dependent resistive and capacitive response of methyl-red thin films in a Ag/methyl-red/Ag surface-type cell. A 300-nm-thick film of methyl-red was deposited from 10 wt% solution in benzene by a spin coater at an angular speed of 2000 revolutions per minute (RPM) on a glass substrate with preliminary deposited metal electrodes to form the Ag/methyl-red/Ag surface-type cell. The length and width of the gap between the electrodes were 50 μm and 15 mm, respectively. The resistance of the film reduced from 37 to 17 MΩ with an elevation of relative humidity level over the whole humidity range. It was also observed that under the effect of humidity, the capacitance of the methyl-red thin film increased by 12 times. The capacitive/resistive sensor has a quasi-linear function with relative humidity in the range of 30–95% and has a small hysteresis. The response and recovery time of the sensor was about 10 s for the capacitive sensor. The humidity-dependent resistive and capacitive properties of this sensor make it promising for use in a humidity meter.

Synthesis of colloidal zinc oxide nanoparticles by pulsed laser ablation in aqueous media

Abstract: Zinc oxide (ZnO) is a promising material for optoelectronic devices and visible–ultraviolet light-emitting diodes. Colloidal ZnO nanocrystals are synthesized by pulsed laser ablation of zinc target in distilled water with passing pure oxygen gas simultaneously. Sodium dodecyl sulfate, C12H25SO4Na, is used as surfactant material to prevent agglomeration and terminates growth of nanoparticles. The UV–visible plasmon absorption, transmission electron microscopy (TEM), XRD and Fourier transform infrared (FTIR) spectroscopy are used for its characterization. Possible mechanism for the synthesis of ZnO and zinc oxyhydroxide nanoparticles are discussed.

Mixed SnO2/TiO2 included with carbon nanotubes for gas-sensing application

Abstract: TiO2 and SnO2 are the well-known sensing materials with a good thermal stability of the former and a high sensitivity of the latter. Carbon nanotubes (CNTs) have also gas sensing ability at room temperature. CNTs-included SnO2/TiO2 material was a new exploration to combine the advantages of three kinds of materials for gas-sensing property. In this work, a uniform SnO2/TiO2 solution was prepared by the sol–gel process with the ratio 3:7 in mole. The CNTs with contents in the range of 0.001–0.5 wt% were dispersed in a mixed SnO2/TiO2 matrix by using an immersion-probe ultrasonic. The SnO2–TiO2 and the CNTs-included SnO2–TiO2 thin films were fabricated by the sol–gel spin-coating method over Pt-interdigitated electrode for gas-sensor device fabrication and they were heat treated at 500 °C for 30 min. FE-SEM and XRD characterizations indicated that the inclusion of CNTs did not affect the particle size as well as the morphology of the thin film. The sensing properties of all as-fabricated sensors were investigated with different ethanol concentrations and operating temperatures. An interesting sensing characteristic of mixed SnO2/TiO2 sensors was that there was a two-peak shape in the sensitivity versus operating temperature curve. At the region of low operating temperature (below 280 °C), the hybrid sensors show improvement of sensing property. This result gives a prospect of the stable gas sensors with working temperatures below 250 °C.

Electromagnetic response of electrons in graphene: Non-linear effects

Abstract: Electrons and holes in graphene have a massless, Dirac energy spectrum. This leads to strongly non-linear electromagnetic response: The graphene layer, irradiated by electromagnetic waves with the frequency Ω , reemits radiation at frequencies m Ω with odd integer m . The graphene based frequency multiplier is tunable and can work in a broad frequency range, in particular, at terahertz frequencies.

Electrolytic synthesis of carbon nanotubes from carbon dioxide in molten salts and their characterization

Abstract: Carbon nanotubes (CNTs) were synthesized from CO2 dissolved in molten salts using the novel electrolytic method developed by the authors. The electrolysis were carried out under current and potential controls. To establish the actual current and potential ranges, the electroreduction of carbon dioxide dissolved in the halide melts under an excess pressure up to 15 bar was studied by cyclic voltammetry on glassy-carbon (GC) electrode at a temperature of 550 °C. The electrochemical–chemical–electrochemical mechanism of CO2 electroreduction was offered for explanation of the obtained results. The structure, morphology, and electronic properties of the CNTs obtained were studied using SEM, TEM, X-ray and electron diffraction analysis, Raman and ESR spectroscopy. It was found that the majority of the CNTs are multi-walled (MWCNTs), have curved form, and most often agglomerate into bundles. Almost all CNTs are filled partly with electrolyte salt. Except MWCNTs the cathode product contains carbon nanofibers, nanographite, and amorphous carbon. The dependences of CNT's yield, their diameter, and structure peculiarities against the electrolysis regimes were established.

Voltage-controlled optical bistability of a tunable three-level system in a quantum-dot molecule

Abstract: We investigate the behavior of optical bistability (OB) in an asymmetry semiconductor double quantum dot (QD) using the tunnel coupling. Such a tunable three-level QD system is driven coherently by a laser field inside the unidirectional ring cavity. The results show that we are able to control efficiently the bistable threshold intensity and the hysteresis loop by tuning the parameters of the system such as the gate voltage and laser frequency. The results obtained can be used for the development of new types of nanoelectronic devices for realizing switching process.

Binding energy of hydrogenic impurity states in an inverse parabolic quantum well under electric field

Abstract: Using the potential morphing method in the effective mass approximation, we have studied the behavior of the impurity binding energy as a function of the impurity position, for different applied electric fields, different Al concentrations at the well center, in strong, intermediate and weak confinement, for a GaAs/Ga 1-x Al x As inverse parabolic quantum well. Our results indicate that the impurity binding energy has the same behavior as the spatial distribution of the electron ground state wavefunction and also that an electron localization appears only in the intermediate and weak confinement regime when the electric field takes non-zero values.

The effects of oxygen partial pressure on the microstructures and photocatalytic property of ZnO nanoparticles

Abstract: The effects of oxygen partial pressure on the microstructures and photocatalytic activity of ZnO nanoparticles prepared by evaporation–condensation method in a flow of Ar and O 2 have been studied by field-emission scanning electron microscopy (FESEM), XRD, XSM800 photoelectron spectrometer (XPS) and UV–Vis spectrometer. The size and shape of ZnO nanoparticles prepared at the oxygen partial pressure (700–800 Pa) show slight difference from that prepared at the oxygen partial pressure ranging from 1500 to 2300 Pa. However, the relative percentage of surface oxygen vacancies decreases from 40.6% (prepared at 700–800 Pa) to 29.3% (2200–2300 Pa) in the condition of similar ratio of Zn atoms to total oxygen species by the qualitative calculation of XPS data. ZnO nanoparticles were examined as photocatalysts for the UV-induced degradation of methyl orange in water solution. The photodegradation results directly demonstrate that the oxygen partial pressure influences the photoactivity of ZnO nanoparticles greatly.

The binding energy of hydrogenic impurity in multilayered spherical quantum dot

Abstract: The binding energy of a hydrogenic impurity of a multilayered spherical GaAs-(Ga,Al)As quantum dot has been investigated as a function of the barrier thickness and the inner dot thickness for various barrier potentials in the effect of the band non-parabolicity. Within the effective mass approximation, the ground state energy has been calculated using the fourth-order Runge–Kutta method. The ground state binding energy of hydrogenic impurity located at the center of a quantum dot has been studied with a variational approach. We have found that a variation in the binding energy has depended on the geometry of the dot.

Electric field effect in a GaAs/AlAs spherical quantum dot

Abstract: Using a variational procedure within the effective-mass approximation we calculate the binding and normalized binding energy (NE bF ) of a shallow donor impurity in a GaAs/AlAs spherical quantum dot, under the action of constant uniform electric field applied in the z -direction. A proper choice of the dot radius and electric field can largely change NE bF of a centre shallow impurity in the spherical quantum dot, which may be used to feel the small change in the dot radius.

Modelling conduction in carbon nanotube networks with different thickness, chemical treatment and irradiation

Abstract: We show how the main features of the electronic transport properties of single-wall carbon nanotube (SWNT) networks can be understood in terms of a simple model involving metallic conduction interrupted by thin tunnelling barriers, backscattering by zone-boundary phonons, and variable range hopping. Within this framework we examine the effect of reducing the thickness of the SWNT networks, chemical treatments, and ion irradiation. The conduction mechanism can be tuned from variable-range hopping between localized states (for the thinnest networks), to metallic conduction interrupted by thin barriers through which conduction is by tunnelling (for thick freestanding films). Chemical treatment of the thick films by different molecules leads to retention of metallic character but changes (increases or decreases) the charge carrier density. For ion-irradiated thick films we find two competing effects: a change to hopping-type conduction in the direct impact layer that lowers conductivity, and annealing effects extending deeper than the ion penetration depth that increase conductivity and lead to a peak in conductivity as a function of irradiation dose. We briefly discuss current–voltage characteristics and possible Luttinger liquid effects.

Raman study on single-walled carbon nanotubes and multi-walled carbon nanotubes with different laser excitation energies

Abstract: In this report, the Raman spectra of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) were studied with different laser excitation energies (325, 514.5 and 1064 nm). The experimental results provide that the relative Raman intensities to the tangential stretching mode (G-mode) of the disorder mode (DM) for SWCNTs ( I D / I G ) hardly decrease with increasing laser excitation energy, but I D / I G of MWCNTs decreases with slope 0.3 eV −1 with increasing laser excitation energy. This may provide important information for the difference of DM between SWCNTs and MWCNTs.

Electronic structure and magnetism in BeO nanotubes induced by boron, carbon and nitrogen doping, and beryllium and oxygen vacancies inside tube walls

Abstract: We have performed ab initio calculations to systematically investigate electronic properties and magnetism of insulating non-magnetic beryllium monoxide nanotubes (BeO NTs) induced by non-magnetic sp impurities: boron, carbon and nitrogen, as well as by nanotube wall defects: Be or O vacancies. We found that in the presence of these sp impurities, which replace oxygen atoms, the non-magnetic BeO NTs transform into magnetic semiconductors, which acquire magnetization caused by spin splitting of (B, C, N) 2p states located in the forbidden gap of a BeO tube. The magnetic moments of the impurities vary from 0.65 to 1.60 μ B . It was also found that a beryllium vacancy leads to vacancy-induced magnetic moments (at about 0.6 μ B ) arising on the nearest oxygen atoms and beryllium-deficient BeO nanotubes adopt half-metallic-like properties. On the contrary, when (B, C, N) dopants substitute for Be atoms or in the presence of an oxygen vacancy, the non-magnetic state of the BeO tubes is retained. The results obtained have been compared with theoretically predicted magnetization effects for doped and non-stoichiometric crystalline BeO.

Synthesis and characterization of Ag/PVA nanorods by chemical reduction method

Abstract: Silver (Ag) nanorods with the average length of 280 nm and diameters of around 25 nm were synthesized by a simple reduction process of silver nitrate in the presence of polyvinyl alcohol (PVA) and investigated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission-electron microscopy (TEM) and UV–vis spectrum. It was found out that both temperature and reaction time are the important factors in determining the morphology and aspect ratios of nanorods. TEM images showed the prepared Ag nanorods have a face centered shape (fcc) with fivefold symmetry consisting of multiply twinned face centered cubes as revealed in the cross-section observations. The fivefold axis, i.e. the growth direction, normally goes along the (1 1 1) zone axis direction of the basic fcc Ag-structure. Preferred crystallographic orientation along the (1 1 1), (2 0 0) or (2 2 0) crystallographic planes and the crystallite size of the Ag nanorods are briefly analyzed.